1. Field of the Invention
This invention relates to a solid propellant rocket motor case for insensitive munitions requirements. The case is made by slotting a metal container, such as a cylinder, and overwrapping the case with fibers, impregnated with resin.
2. Description of the Prior Art
Military operations and other storage environments provide a potential for undesired heat, such as from a fire, in the vicinity of rocket motors in storage or in use prior to a normal firing. An exposed rocket motor can react to exposure to heat and/or flames in a short time. The undesired heat and/or flames can cause a violent explosion or uncontrolled thrust due to autoignition of propellant resulting in the rocket firing but being out of control.
The burning of a propellant material within the case produces gases which are controllably released through a nozzle to produce thrust for propelling a rocket motor during normal firing. The propellant material is ignited for such use by an igniter system. However, each type of propellant material has an autoignition temperature, that is, a temperature at which the propellant will automatically ignite. The propellant material for a particular rocket motor is thus selected to have an autoignition temperature which is substantially greater than the ambient or normal temperature range for storage of the rocket motor prior to its use. If, due to an unexpected fire near a stored rocket motor, the propellant material in the rocket motor becomes so hot that it autoignites, not only may the fire hazard become increased but the rocket motor may become propulsive thus magnitudinally increasing the catastrophic damage which may occur. Also, while such a danger of propulsion may not be prevalent with other kinds of gas generators, it would still be desired to prevent catastrophic burst hazards from a high buildup of pressure within any gas generator.
Past efforts to improve the heat resisting capability of ordnance items have included placing a thermal barrier on the exterior of the rocket motor casing or warhead. By thermal insulation of the rocket motor casing or explosive, the length of time the ordnance item can be exposed to fire without reaction is increased. If the fire is not extinguished within a short period of time, the internal temperature will increase and the ordnance item may ignite and explode.
Explosion and violent rupture of a heat weakened motor can occur when the propellant grain is ignited along the central void in the grain. If combustion can be limited to the outside of the grain and properly vented, the severity of the reaction is lessened.
U.S. Pat. No. 4,458,482 addresses the problem by carefully aligning bare patches in the insulating coating of the propellant shaped to coincide with stress patterns to aid venting.
U.S. Pat. No. 4,442,666 uses carefully aligned gaps in the insulation along the inside of the casing carefully arranged so as not to be opposite radial grooves in the propellant, causing rupture of the casing to vent the propellant at the specially placed gaps in the insulation.
U.S Pat. No. 4,411,199 has a metallic cap for a missile fuse booster with apertures covered by an inner cap of a thermoplastic which melts to vent through the aperture and prevent the booster charge from exploding at higher temperatures, when not in actual use.
U.S. Pat. No. 4,838,166 similarly has bores filled with meltable material to vent an explosive charge at prolonged undesired higher than ambient temperatures.
Finally, U.S. Pat. No. 5,060,470 discloses a ventable rocket motor or other gas generator case which is ventable at a temperature between ambient and autoignition for the propellant by memory metal members interleaved between plies of the composite of the casing.